The object of the experiments proposed here is to examine the regulation of expression of idiotype-bearing antibody by regulatory sets of T cells. Our approach will build on previous experience with the antibody response to the haptenic determinant 2,4-dinitrophenyl (DNP). This response is known to be controlled by at least two distinct types of helper T cells, one of which requires immunoglobulin (Ig) or B cells for its activation. This latter helper T cell is critical for the expression of germ line encoded idiotypic antibody. These helper T cells will be characterized in terms of their specificity both for idiotype and for antigen, their surface expression of alloantigenic determinants (Lyt, Ia and Qa antigens), and the requirements for their activation by antigen and Iq. Restriction of the behavior of these helper T cells by self major histocompatibility antigens will also be determined. As part of this study, the genetic basis of the expression of the idiotype in question will be determined. We will also attempt to determine whether this idiotype is expressed in random fashion on all Ig isotypes, or whether its expression is predominant in some isotypes and not in others. The latter result would imply that idiotype and isotype can be co-regulated by a single mechanism. Finally, means by which the expression of idiotype can be suppressed will be explored. From this study of the regulation of idiotype expression by T cells, we hope to construct a general model for immunoregulation, with particular relevance to the problem of the regulation of expression of antibody molecules with different Ig isotypes. A second aspect of the current work will be to use anti-idiotypic antibody to explore the genetic and cellular basis of idiotype expression by T cells. T cells specific for DNP will be tested for their expression of idiotype. Anti-idiotypic antibody will be used to determine if the expression of idiotypic determinants by T cells is highly variable, or whether T cells express only a limited range of idiotypic determinants. Such antibodies should also allow the isolation and molecular characterization of various T cell antigen binding receptors. For these studies, clones of hapten-reactive T cells will also be generated using T cell growth factors so that homogeneous populations of T cells can be assessed for idiotype expression.